test: set par=2 in default config for functional test framework

Depending on the host machine, a default `par` value can spawn up to 15 script verification threads for each node.
Running the functional test suite with default `par` can exhaust file descriptors or hit other resource limits when many threads are spawned.
These threads are mostly idle and the same code paths are executed with a value of `par=2`.
Limit this to 2 for functional tests that do not override the default option.

Co-authored-by: maflcko <6399679+maflcko@users.noreply.github.com>

test/functional/test_framework/util.py

@@ -474,6 +474,7 @@ def write_config(config_path, *, n, chain, extra_config="", disable_autoconnect=
         #  min_required_fds = MIN_CORE_FDS + MAX_ADDNODE_CONNECTIONS + nBind = 151 + 8 + 3 = 162;
         #  nMaxConnections = available_fds - min_required_fds = 256 - 161 = 94;
         f.write("maxconnections=94\n")
+        f.write("par=" + str(min(2, os.cpu_count())) + "\n")
         f.write(extra_config)